In order to realize a large-area and highly-precise next-generation display, a transistor having a high mobility is required. For use in a transistor, an oxide semiconductor has attracted attention. As a candidate material of an oxide semiconductor, a thin film of amorphous indium gallium zinc oxide (IGZO) is promising. This thin film can be obtained by sputtering a target formed of a sintered body having the same composition.
In a sputtering method, generally, glow discharge is allowed to occur between a substrate as the anode and a target as the cathode in an argon gas pressure of about 10 Pa or less, thereby to generate argon plasma. Argon positive ions in this plasma collide with the target as the cathode to cause the particles of the target components to fly off, and these particles are then deposited on the substrate to form a film.
A sputtering method is divided into several methods according to the manner of argon plasma generation. A method which uses high-frequency plasma is called high-frequency sputtering method, and a method which uses direct current plasma is called direct current sputtering method. Further, a method in which magnets are arranged on the rear side of the target to allow argon plasma to concentrate on just above the target, whereby collision efficiency of argon ions is increased to enable film formation at a low pressure is called “magnetron sputtering method”.
An IGZO sintered body is generally used as an oxide semiconductor target. An IGZO sintered body is produced by a powder sintering method in which, substantially, an indium oxide, a gallium oxide and a zinc oxide are compounded to have a desired composition, and the resulting mixture is subjected to press shaping, and the shaped body is then sintered at a temperature of 1400° C. or higher (Patent Document 1).
Further, in order to improve transistor performance, development of a sintered body for obtaining a crystalline oxide semiconductor such as indium oxide gallium (IGO) has been studied (Patent Document 2). As for the method for sintering an IGO target, by conducting sintering at 1200° C. to 1600° C. for 2 hours or more, gallium is solid dissolved in an indium site, whereby a high-density target can be obtained.
However, it has been found that if sputtering is conducted at a large power of 3 W/cm2 or more by using a middle- to large-sized target in which a longer side or one side has a length exceeding 5 inches (i.e. a target having an area exceeding 25000 mm2) that is produced under these conditions, particles may generate.
Also, it has been confirmed that such a phenomenon hardly occurs in a small-sized target having a size of 4 inches or less. An oxide semiconductor is originally a material that meets requirements for an increase in area of a display. Of these oxide semiconductors, a material comprising both indium and gallium such as IGO has a problem that particles may generate. Therefore, there is a demand for solving this problem.